1. Field of the Invention
The present invention relates to a potential measuring device of a non-contact type for measuring a potential of a measurement object based on a quantity of charge induced on a detection electrode, and, also, to an image forming apparatus having the potential measuring apparatus, which can be applied to a copying machine or a printer.
2. Related Background Art
In the prior art, for example, in an image forming apparatus having a photosensitive drum for conducting image formation by electrophotography, in order to constantly obtain a stable image quality, it is necessary to keep the potential of the photosensitive drum to be appropriately (generally uniformly) charged in any environment. For that reason, a potential measuring device is used to measure the charge potential of the surface of a photosensitive drum, and the result of the measurement is used to perform feedback control to thereby keep the potential of the photosensitive drum uniform.
The conventional potential measuring device includes a non-contact type potential measuring device, in which a system called a mechanically modulated alternating electrical field induction type is frequently used. In this system, the electrical potential of a surface of a measurement object is expressed as a function of the magnitude of a current i extracted from a detection electrode incorporated into the potential measuring device. The electrical potential is expressed by the following expression.
                    i        =                                            ⅆ              Q                                      ⅆ              t                                =                                    ⅆ                              ⅆ                t                                      ·                          (                              C                ·                V                            )                                                          (        1        )            In the above expression, Q is a charge quantity that develops on the detection electrode, C is a coupled capacitance between the detection electrode and the measurement object, and V is a surface potential of the measurement object. Also, the capacitance C is expressed by the following expression.
                    C        =                              A            ·            S                    x                                    (        2        )            
In the above expression, A is a proportional constant related to a dielectric constant of a material, and so on, S is an area of the detection electrode, and x is a distance between the detection electrode and the measurement object.
Those relationships are used to measure the potential V of the surface of the measurement object. It is known that it is preferable to cyclically modulate the magnitude of the capacitance C between the detection electrode and the measurement object, in order to accurately measure the charge quantity Q that develops on the detection electrode. That is, the charge quantity Q that develops on the detection electrode has such a small value as to be susceptible to noises that exist around the detection electrode. Therefore, in order to accurately measure a charge quantity Q having a small quantity, a synchronous detection system is often used, in which the magnitude of the coupled capacitance C between the detection electrode and the measurement object is cyclically modulated by an appropriate means, and components of the same frequency are detected from the measured signal, to thereby obtain a necessary signal.
As a method of modulating the capacitance C, the following methods are known.
U.S. Pat. No. 4,720,682 discloses a method in which a grounded fork-shaped shutter is inserted between the measurement object and the detection electrode, and the shutter cyclically moves in a direction parallel to the surface of the measurement object to thereby realize modulation of the coupled capacitance C between the measurement object and the detection electrode.
U.S. Pat. No. 3,852,667 discloses a structure in which a metal shield material having an aperture is disposed at a position facing the measurement object, and a detection electrode is disposed at a leading edge of a fork-shaped oscillating element, to thereby change the position of the detection electrode immediately below the aperture in a parallel direction. With the above structure, the number of electrical lines of force reaching the detection electrode is modulated, and the electrostatic capacitance C is modulated.
On the other hand, in order to downsize the electrophotography image forming apparatus, it is necessary to downsize the photosensitive drum and to increase packaging density around the drum, and the potential measuring device is also required to be made more compact and thinner. However, in the above-mentioned sensor of the mechanically modulated alternating electrical field induction type, assembling parts such as a driving mechanism for oscillating the fork-shaped shutter or the fork-shaped oscillating body take up most of the internal space of the sensor structure. Accordingly, it is necessary to downsize those driving mechanisms in order to downsize the potential measuring device.
Along with an increase in the demand for the above downsizing, in recent years, there has been reported an attempt to form a micro mechanical structure on a semiconductor substrate by using a semiconductor processing technology called “micro electro mechanical system (MEMS)” technology. There has also been reported a mechanically modulated alternating electrical field induction type potential measuring device using the above technology. As a typical example thereof, U.S. Pat. No. 6,177,800 discloses a device in which a shutter structure having a minute aperture, which has been manufactured through the semiconductor processing technology, is oscillated immediately above a detection electrode to measure the potential of a measurement object.
In the potential measuring device of the mechanically modulated alternating electrical field induction type using the above conventional art, in order to oscillate a fork-shaped shutter or a shutter having an aperture formed therein just above a detection electrode, an electromagnetic force, a generative force due to a piezoelectric element, or an electrostatic force is frequently used.
In the general potential measuring device described in the above example, a piezoelectric element is attached onto a fork-shaped shutter, and the fork-shaped shutter is oscillated by using the mechanical oscillation generated by a voltage which is applied to the piezoelectric element. At this time, an AC voltage of several volts to several tens of volts, or higher, is applied to the piezoelectric element, which causes AC noises to be generated in the detection electrode and to be superimposed on the detection signal. The noise frequently has the same frequency component as that of the detection signal, so it is not easy to remove the noise from the detection signal.
Likewise, in the system of oscillating the above-mentioned fork-shaped shutter with the use of an electromagnet, the detection signal is susceptible to noises generated by a current for driving the electromagnet. In addition, in the MEMS technology disclosed in the above example, for example, the modulated voltage of about several volts to one hundred and several tens of volts is applied to the oscillation mechanism, called a “comb-shaped electrostatic actuator,” to oscillate the shutter. In the element using the MEMS technology, there are many cases in which, in order to downsize the device, the actuator member and the detection electrode are disposed close to each other such that the distance therebetween is, for example, 1 mm or shorter. Therefore, the detection signal is greatly susceptible to noises generated by the driving signal.